Historical Constraints and the Evolution of Development 1027
of "constraint" as promoting change in particular directions marks the positive
meaning that motivates this chapter and raises an important issue for Darwinian
theory in providing a best general case and opportunity for positive interaction with
the functionalist precepts of natural selection.
The first (empirical) positive meaning of channeling
Orthodox Darwinian functionalists have often reacted by arguing "why the big fuss,
we know (and both admit and use) the concept of constraint already" to evolutionists
of a structuralist bent who claim that a properly formulated version of "constraint"
should evoke great interest and provoke substantial reform. I would accept this jaded
reaction if the version of "constraint" proffered by structuralist critics stayed within
the negative meanings described above. But positive meanings of constraint—and I
will outline two different constructions of positivity in this section—can lead to
important extensions of evolutionary theory by questioning and reformulating what I
have called the second branch, or the second tripod leg, of Darwinism's essential triad
of indispensable arguments: the functionalist attribution of effectively all substantial
evolutionary change to natural selection.
I agree that negative constructions of constraint do not seriously challenge this
major precept (while supplying some interesting subtleties and wrinkles that orthodox
functionalists can use and appreciate) for the following set of interconnected reasons:
Mainline Darwinism is a functionalist theory of "trial-and-error externalism" (in R. C.
Lewontin's phrase). The organism "proposes" by generating variation, ultimately by
mutation (and subsequently by distribution in sexual recombination for organisms
traditionally deemed "higher"), among members of populations. This variation acts as
raw material—the "chance" in Monod's famous metaphor of "chance and necessity";
the "error" in Lewontin's "trial-and-error"—for a causal process of natural selection
(the "necessity" in Monod's pairing; the "trial" of Lewontin's joining). That is, the
organism proposes, and the environment (interacting with the organism) disposes.
The organism's generation of variation provides the internal component of
evolution; the environment's process of selection marks the external contribution.
These internal and external factors play strikingly different roles in Darwinian
theory—a contrast well epitomized (as noted above) in Monod's phrase "chance and
necessity." The internal component can only supply raw material and does not
establish the rates or vectors of change. This claim— that variation provides
potential, but not direction—sets a fundamental postulate of Darwinian mechanics
and philosophy. Natural selection, the external component, carries full responsibility
for the direction—and also, ultimately, for the modes and rates—of evolutionary
change.
As discussed at length in Chapter 2, Darwin's central insight that variation must
be "isotropic"—particularly, that it be copious in amount, small in extent, and
undirected towards adaptive configurations—underlies his brilliant grasp of what
selection requires from variation to permit a functionalist theory to operate in
principle, and also to dominate the causes of evolutionary change. If variation is truly
isotropic in Darwin's hypothesized sense, then selection